1. Field of the Invention
This invention relates to determining optical wavelengths. More particularly, this invention relates to determining optical wavelengths using a plurality of resolvable features, such as spectral peaks and troughs, of a multi-component spectrum, such as those produced by multiple Bragg gratings or by a super-structured Bragg grating.
2. Description of the Related Art
Accurately determining the operational wavelength of an optical component such as a fiber Bragg grating (FBG) sensor is often very important. Some applications improve measurement accuracy by performing a statistical analysis (e.g. averaging) on a number of repeated optical measurements of the same device. This however increases the overall measurement time.
An FBG element is an optical element that is usually formed by photo-induced periodic modulation of the refractive index of an optical fiber's core. An FBG element is highly reflective to light having wavelengths within a narrow bandwidth that is centered at a wavelength that is referred to as the Bragg wavelength. Other wavelengths are passed through the FBG without reflection. The Bragg wavelength itself is dependent on physical parameters, such as temperature and strain, that impact on the refractive index. Therefore, FBG elements can be used as sensors to measure such parameters. After proper calibration, the Bragg wavelength is an absolute measure of the physical parameters. While the foregoing has described a single Bragg grating, multiple Bragg gratings or a super structured fiber Bragg grating can be formed at a particular position.
While FBG elements make useful sensors, it is very important to accurately measure the Bragg wavelength. Indeed, an accuracy and repeatability of less than 1 pm can be required.
Bragg wavelengths are found by sweeping light across a bandwidth that includes all of the possible Bragg wavelengths, and by measuring the power (intensity) of the reflected light over time. This is typically performed using optical sensors and optical couplers, a broadband light source, for example an edge-light-emitting diode (ELED) or a superfluorescent fiber source (SFS), and a tunable optical filter, for example a piezoelectric transducer (PZT) tunable fiber Fabry-Perot filter [Kersey, A. D., Berkoff, T. A., and Morey, W. W., “Multiplexed Fiber Bragg Grating Strain-Sensor System With A Fiber Fabry-Perot Wavelength Filter”, Optics Letters, Vol. 18, pp. 1370–1372, 1993]. Alternatively, optical sensors and optical couplers can be used with a tunable laser source. For example an external cavity semiconductor laser with a tunable FBG reflector, reference, for example, U.S. Pat. No. 5,401,956, issued on Mar. 28, 1995.
A portion of the light from the optical source is coupled to an accurate wavelength reference element, such as a fixed Fabry-Perot wavelength filter, and the transmitted or reflected (as appropriate) light power is also measured over time. By comparing the measured power signal from the sensor elements against that received from the accurate wavelength reference accurate Bragg wavelengths of the FBGs sensors can be determined. Then, by noting the change in the Bragg wavelength from an unstressed condition a physical parameter of interest can be found, e.g., the temperature or pressure proximate the FBG can be determined. Many FBG elements can be located along one or more optical fibers providing multiple sensors to be demodulated by a single instrument.
The ultimate measurement resolution of such systems is limited by a variety of noise sources and by the received optical power from each element. Current measurement systems continue to push both the source and receiver design to their limits; high power lasers, low-noise, high sensitivity receivers, advanced signal processing. Yet one area that has received less attention is in the design of the optical element itself, through which, as shown by this invention, significant performance gains can be realized.